BACKGROUND Non-responders in questionnaire surveys may introduce bias and lower the validity of the studies. A Cochrane review has identified trials evaluating different ways of increasing response rates (1). The odds of responding on postal questionnaires were almost doubled using monetary incentive, and the odds were higher when the incentive was given with the questionnaires compare to only given after participants had responded. However, to our knowledge none of the studies concerned vulnerable children and youth. The purpose of the study is to investigate if a monetary incentive can increase the response rate for vulnerable children and youth in a postal survey. A secondary purpose is to test if monetary incentive has an effect on the content of answers in a postal survey.